This invention is related to borosilicate glasses and in particular to lead- and arsenic-free borosilicate glasses. In another aspect, this invention is related to glass envelopes for electric lamps.
In recent years, the glass industry has been moving toward the elimination of lead and arsenic from borosilicate glass compositions. For example, commonly-owned U.S. Pat. No. 6,284,686, which is incorporated herein by reference, describes a useful lead- and arsenic-free borosilicate glass composition for use in electric lamps (OSRAM SYLVANIA Type SG773). Other lead- and arsenic-free borosilicate glasses used to manufacture electric lamps include types 8486 and 8487 manufactured by Schott Glas.
While the elimination of lead and arsenic is beneficial to the environment, their elimination can result in an increase in the number of glass defects, particularly knots, stones, and cord. It is believed that the majority of the defects which occur in lead- and arsenic-free borosilicate glasses are caused by incomplete melting and a reduced convective mixing of the batch components, e.g., knots and stones are the remnants of incomplete melting and surface scum. Therefore, it would be desirable to improve the melting characteristics of lead- and arsenic-free borosilicate glasses to reduce the number of glass defects.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to reduce the number of glass defects which occur when melting lead- and arsenic-free borosilicate glasses.
It is yet another object of the invention to improve the melting characteristics of lead- and arsenic-free borosilicate glasses.
These and other objects and advantages of the invention are achieved by substantially eliminating MgO and CaO from the borosilicate glass formulation and adding from 1.0 to 4.0 weight percent of BaO. In particular, there is provided a lead- and arsenic-free borosilicate glass having a composition which comprises in weight percent:
wherein the sum of Na2O, K2O and Li2O is from 4.7 to 6.5 weight percent and the composition is substantially free of MgO and CaO.
Preferably, the borosilicate glass of this invention has a BaO content from 2.0 weight percent to 4.0 weight percent, and more preferably, 2.0 weight percent.
In a narrower aspect, the lead- and arsenic-free borosilicate glass of this invention has a composition which comprises in weight percent:
wherein the sum of Na2O, K2O and Li2O is from 4.7 to 6.5 weight percent and the composition is substantially free of MgO and CaO.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims.
Glass defects resulting from incomplete melting and reduced convective mixing tends to be more of a problem for gas-fired melters as compared to electric glass furnaces. Gas-fired melters produce glass melts with molten surface layers and rely on convective currents in the melt for mixing the molten surface with the bulk. Electric glass furnaces are usually cold crowned so that gravity acts to pull the cooler, denser glass at the surface into the melt. The cooler surface also reduces volatilization and surface scum formation. Therefore, although the glass of this invention may be melted in most conventional glass melters, including electric furnaces, the improved melting characteristic it provides is particularly advantageous for gas-fired melters as described herein.
During the melting process in a gas-fired melter, fining gases can float refractory grains (primarily sand grains) to the surface of the melt, resulting in density gradients. An accumulation of sand grains at the melt surface will be difficult to mix with the underlying, higher density glass. A silica-rich surface scum can result. To prevent scum formation, convective forces are needed to move sand grains back into the melt to complete the melting process. With gas-fired melters, it is common practice to install a single or dual bubbler row in the melter to assist convective mixing. This method is only partially effective in preventing silica scum.
Lowering melt surface tension of the borosilicate glass was investigated as a means to improve convective mixing. Melting experiments have shown that with lead-and arsenic-containing borosilicate glasses, convective mixing is quite vigorous. Both PbO and As2O3 lower melt surface tension. BaO is also known to lower the surface tension of glass.
However, other alkaline earths, such as MgO, CaO, and SrO are all known to increase surface tension. (See, e.g., W. A. Weyl and E. C. Marboe, The Constitution of Glasses: A Dynamic Interpretation, Vol II, (1964), pp 639-644; and M. B. Volf, Chemical Approach to Glass, Elsevier, (1984)). It was discovered that the melting and fining characteristics of the Type SG773 lead- and arsenic-free borosilicate glass described in U.S. Pat. No. 6,284,686 could be dramatically improved when MgO and CaO were removed from the glass composition and from 1.0-4.0 weight percent (wt. %) BaO added. In particular, ninety-nine percent fewer stones (unmelted raw material particles) have been observed compared to a typical SG773 glass. This dramatic improvement is believed to be the result of better convective mixing in the melt caused by lowering the melt""s surface tension.
It is common practice for manufacturers of prior art borosilicate glass compositions to use dolomite, CaMg (CO3)2, as an inexpensive source of MgO and CaO. Dolomite is not used in the borosilicate composition of this invention since MgO and CaO are not contained in the glass composition. However, although CaO are MgO are not purposefully added to the borosilicate glass of this invention, small amounts of CaO and MgO cannot be completely excluded since Mg and Ca impurities may be present in some of the naturally occurring batch materials. Therefore, as used herein, substantially free of MgO and CaO means that the aggregate amounts of MgO and CaO in the glass composition do not exceed about 0.15 weight percent (wt. %).
Both anhydrous borax (ABX) and 5-mol borax (BX), Na2O.B2O3. 5H2O, have been used as sources of B2O3 and Na2O in borosilicate glass compositions. Laboratory experiments have shown that silica grains are floated the most when 5-mol borax is used as the borax containing raw material. In particular, the greatest improvement in the melting rate observed thus far was for a batch containing 4 wt. % BaO (no dolomite) melted with a 5-mol borax.